Evolutionary psychology (EP) is an approach to the entire discipline that views [[human nature]] as a universal set of evolved psychological adaptations to recurring problems in the ancestral environment. Proponents of EP suggest that it seeks to heal a fundamental division at the very heart of science --- that between the [[Soft science|soft]] human [[social science]]s and the [[Hard science|hard]] [[natural science]]s, and that the fact that human beings are living organisms demands that [[psychology]] be understood as a branch of [[biology]]. Anthropologist [[John Tooby]] and psychologist [[Leda Cosmides]] note:

Evolutionary psychology (EP) is an approach to the entire discipline that views [[human nature]] as a universal set of evolved psychological adaptations to recurring problems in the ancestral environment. Proponents of EP suggest that it seeks to heal a fundamental division at the very heart of science --- that between the [[Soft science|soft]] human [[social science]]s and the [[Hard science|hard]] [[natural science]]s, and that the fact that human beings are living organisms demands that [[psychology]] be understood as a branch of [[biology]]. Anthropologist [[John Tooby]] and psychologist [[Leda Cosmides]] note:

Evolutionary psychologists argue that much of human behavior is generated by psychological adaptations that evolved to solve recurrent problems in human ancestral environments. They hypothesize, for example, that humans have inherited special mental capacities for acquiring language, making it nearly automatic, while inheriting no capacity specifically for reading and writing. Other adaptations, according to EP, might include the abilities to infer others' emotions, to discern kin from non-kin, to identify and prefer healthier mates, to cooperate with others, and so on. Consistent with the theory of natural selection, evolutionary psychology sees organisms as often in conflict with others of their species, including mates and relatives. For example, mother mammals and their young offspring sometimes struggle over weaning, which benefits mother more than the child. Humans, however, have a marked capacity for cooperation as well.

Evolutionary psychologists see those behaviors and emotions that are nearly universal, such as fear of spiders and snakes, as more likely to reflect evolved adaptations. Evolved psychological adaptations (such a the ability to learn a language) interact with cultural inputs to produce specific behaviors (e.g., the specific language learned). This view is contrary to the idea that human mental faculties are general-purpose learning mechanisms.

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Evolutionary psychology (EP) is an approach to the entire discipline that views human nature as a universal set of evolved psychological adaptations to recurring problems in the ancestral environment. Proponents of EP suggest that it seeks to heal a fundamental division at the very heart of science --- that between the soft human social sciences and the hardnatural sciences, and that the fact that human beings are living organisms demands that psychology be understood as a branch of biology. Anthropologist John Tooby and psychologist Leda Cosmides note:

"Evolutionary psychology is the long-forestalled scientific attempt to assemble out of the disjointed, fragmentary, and mutually contradictory human disciplines a single, logically integrated research framework for the psychological, social, and behavioral sciences—a framework
that not only incorporates the evolutionary sciences on a full and equal basis, but that systematically works out all of the revisions in existing belief and research practice that such a synthesis requires."[1]

In the distant future I see open fields for far more important researches. Psychology will be based on a new foundation, that of the necessary acquirement of each mental power and capacity by gradation. Light will be thrown on the origin of man and his history.—Charles Darwin, On the Origin of Species

Evolutionary psychology is a hybrid discipline that draws insights from modern evolutionary theory, biology, cognitive psychology, anthropology, economics, computer science, and paleoarchaeology. The discipline rests on a foundation of core premises. According to evolutionary psychologist David Buss, these include:

Manifest behavior depends on underlying psychological mechanisms, information processing devices housed in the brain, in conjunction with the external and internal inputs that trigger their activation.

Evolution by selection is the only known causal process capable of creating such complex organic mechanisms.

Selection designed the information processing of many evolved psychological mechanisms to be adaptively influenced by specific classes of information from the environment.

Human psychology consists of a large number of functionally specialized evolved mechanisms, each sensitive to particular forms of contextual input, that get combined, coordinated, and integrated with each other to produce manifest behavior.

Similarly, pioneers of the field Leda Cosmides and John Tooby consider five principles to be the foundation of evolutionary psychology:

The brain is a physical system. It functions as a computer with circuits that have evolved to generate behavior that is appropriate to environmental circumstances

Neural circuits were designed by natural selection to solve problems that human ancestors faced while evolving into Homo sapiens

Consciousness is a small portion of the contents and processes of the mind; conscious experience can mislead individuals to believe their thoughts are simpler than they actually are. Most problems experienced as easy to solve are very difficult to solve and are driven and supported by very complicated neural circuitry

Different neural circuits are specialized for solving different adaptive problems.

Evolutionary psychology is founded on the computational theory of mind, the theory that the mind, our "inner world," is the action of complex neural structures in the brain. For example, when a child shrinks in fear from a spider, the child's brain has attended to the spider, computed that it's a potential threat, and initiated a fear response.

Evolutionary psychology is rooted in evolutionary theory. It is sometimes seen not simply as a sub-discipline of psychology but as a way in which evolutionary theory can be used as a meta-theoretical framework within which to examine the entire field of psychology.[5] A few biologists challenge the basic premises of evolutionary psychology.[6]

Natural selection, a key component of evolutionary theory, involves three main ingredients:

Genetically based inheritance of traits - some traits are passed down from parents to offspring in genes,

Variation - heritable traits vary within a population (now we know that mutation is the source of this genetic variation),

Differential survival and reproduction - these traits will vary in how strongly they promote the survival and reproduction of their bearers.

Selection refers to the process by which environmental conditions "select" organisms with the appropriate traits to survive; these organisms will have such traits more strongly represented in the next generation. This is the basis of adaptive evolution. Darwin's great claim was that this "natural selection" was creative - it could lead to new traits and even new species, it was centred on individual survival, and it could explain the broad scale patterns of evolution.

Many traits that are selected for can actually hinder survival of the organism while increasing its reproductive opportunities. Consider the classic example of the peacock's tail. It is metabolically costly, cumbersome, and essentially a "predator magnet." What the peacock's tail does do is attract mates. Thus, the type of selective process that is involved here is what Darwin called "sexual selection." Sexual selection can be divided into two types:

Intersexual selection, which refers to the traits that one sex generally prefers in the other sex, (e.g. the peacock's tail).

Intrasexual competition, which refers to the competition among members of the same sex for mating access to the opposite sex, (e.g. two stags locking antlers).

Inclusive fitness theory, which was proposed by William D. Hamilton in 1964 as a revision to evolutionary theory, is basically a combination of natural selection, sexual selection, and kin selection. It refers to the sum of an individual's own reproductive success plus the effects the individual's actions have on the reproductive success of their genetic relatives. General evolutionary theory, in its modern form, is essentially inclusive fitness theory.

Inclusive fitness theory resolved the issue of how "altruism" evolved. The dominant, pre-Hamiltonian view was that altruism evolved via group selection: the notion that altruism evolved for the benefit of the group. The problem with this was that if one organism in a group incurred any fitness costs on itself for the benefit of others in the group, (i.e. acted "altruistically"), then that organism would reduce its own ability to survive and/or reproduce, therefore reducing its chances of passing on its altruistic traits. Furthermore, the organism that benefited from that altruistic act and only acted on behalf of its own fitness would increase its own chance of survival and/or reproduction, thus increasing its chances of passing on its "selfish" traits.
Inclusive fitness resolved "the problem of altruism" by demonstrating that altruism can evolve via kin selection as expressed in Hamilton's rule:

cost < relatedness × benefit

In other words, altruism can evolve as long as the fitness cost of the altruistic act on the part of the actor is less than the degree of genetic relatedness of the recipient times the fitness benefit to that recipient.
This perspective reflects what is referred to as the gene-centered view of evolution and demonstrates that group selection is a very weak selective force. However, in recent years group selection has been making a comeback, (albeit a controversial one), as multilevel selection, which posits that evolution can act on many levels of functional organization, (including the "group" level), and not just the "gene" level.

Organisms can evolve physical and mental traits designed specifically to attract mates (e.g., the Peacock’s tail) or to compete with members of one’s own sex for access to the opposite sex (e.g., antlers).

In most species with pronounced sexual selection, the adaptations are in males. These adaptations tend to evolve in species in which a successful male mates with multiple females. For instance, they appear in peacocks but not raptors, which are generally monogamous. Females rarely evolve such adaptations because being the "top female" doesn't improve a female's reproductive career as much as being "top man" improves a man's reproductive outcome.

Peacock’s tail, antlers, courtship behavior, etc

System Level:Family & Kin

Problem:

Gene replication. How to help those with whom we share genes survive and reproduce?

Selection occurs most robustly at the level of the gene, not the individual, group, or species. Reproductive success can thus be indirect, via shared genes in kin. Being altruistic toward kin can thus have genetic payoffs. (Also see Gene-centered view of evolution)
Also, Hamilton argued that sexual reproduction evolved primarily as a defense against pathogens (bacteria & viruses) to "shuffle genes" to create greater diversity, especially immunological variability, in offspring.

Altruism toward kin, parental investment, the behavior of the social insects with sterile workers (e.g., ants).

System Level:Non-kin small group

Problem:How to succeed in competitive interactions with non-kin? How to select the best strategy given the strategies being used by competitors?

The two sexes often have conflicting strategies regarding how much to invest in offspring, and how many offspring to have.

Parents allocate more resources to their offspring with higher reproductive value (e.g., "mom always liked you best"). Parents and offspring may have conflicting interests (e.g., when to wean, allocation of resources among offspring, etc.).

Sexually dimorphic adaptations that result in a "battle of the sexes," parental favoritism, timing of reproduction, parent-offspring conflict, sibling rivalry, etc.

System Level:

Non-kin small group
Problem:

How to succeed in competitive interactions with non-kin? How to select the best strategy given the strategies being used by competitors?

Organisms adapt, or respond, to competitors depending on the strategies used by competitors. Strategies are evaluated by the probable payoffs of alternatives. In a population, this typically results in an "evolutionary stable strategy," or "evolutionary stable equilibrium" -- strategies that, on average, cannot be bettered by alternative strategies.

(Also called "strong reciprocity"). One can play nice with non-kin strangers even in single interactions if social rules against cheating are maintained by neutral third parties (e.g., other individuals, governments, institutions, etc.), a majority group members cooperate by generally adhering to social rules, and social interactions create a positive sum game (i.e., a bigger overall "pie" results from group cooperation).

Generalized reciprocity may be a set of adaptations that were designed for small in-group cohesion during times of high inter-tribal warfare with out-groups.

Today the capacity to be altruistic to in-group strangers may result from a serendipitous generalization (or "mismatch") between ancestral tribal living in small groups and today's large societies that entail many single interactions with anonymous strangers. (The dark side of generalized reciprocity may be that these adaptations may also underlie aggression toward out-groups.)

Genes are not the only replicators subject to evolutionary change. “Memes” (e.g., ideas, rituals, tunes, cultural fads, etc.) can replicate and spread from brain to brain, and many of the same evolutionary principles that apply to genes apply to memes as well. Genes and memes may at times co-evolve ("gene-culture co-evolution").

Middle-level evolutionary theories are theories that encompass broad domains of functioning. They are compatible with general evolutionary theory but not derived from it. Furthermore, they are applicable across species. During the early 1970s, three very important middle-level evolutionary theories were contributed by Robert Trivers:[8][9][10]

The theory of reciprocal altruism explains how altruism can arise amongst non-kin, as long as there is a sufficient probability of the recipient of the altruistic act reciprocating at a later date. The possibility was also noted by Trivers, later coined 'indirect altruism' by Richard D. Alexander, that reciprocation could be provided by third parties, raising the issue of social reputation.

Parental investment theory refers to the different levels of investment in offspring on the part of each sex. For example, females in any species are defined as the sex with the larger gamete. In humans, females release approximately one large, metabolically costly egg per month, as opposed to the millions of relatively tiny and metabolically cheap sperm that are produced each day by males. Females are fertile for only a few days each month, while males are fertile every day of the month. Females also have a nine month gestation period, followed by a few years of lactation. Males' obligatory biological investment can be achieved with one copulatory act. Consequently, human females have a significantly higher obligatory investment in offspring than males do. (In some species, the opposite is true.) Because of this difference in parental investment between males and females, the sexes face different adaptive problems in the domains of mating and parenting. Therefore, it is predicted that the higher investing sex will be more selective in mating, and the lesser investing sex will be more competitive for access to mates. Thus, differences in behaviour between sexes is predicted to exist not because of maleness or femaleness per se, but because of different levels of parental investment.

The theory of parent-offspring conflict rests on the fact that even though a parent and his/her offspring are 50% genetically related, they are also 50% genetically different. All things being equal, a parent would want to allocate their resources equally amongst their offspring, while each offspring may want a little more for themselves. Furthermore, an offspring may want a little more resources from the parent than the parent is willing to give. In essence, parent-offspring conflict refers to a conflict of adaptive interests between parent and offspring. However, if all things are not equal, a parent may engage in discriminative investment towards one sex or the other, depending on the parent's condition.

Additional middle-level evolutionary theories used in EP include:

The Trivers-Willard hypothesis, which proposes that parents should invest more in the sex that gives them the greatest reproductive payoff (grandchildren) with increasing or marginal investment. Females are the heavier parental investors in our species. Because of that, females have a better chance of reproducing at least once in comparison to males. Thus, according to the Trivers-Willard hypothesis, parents in good condition are predicted to favor investment in sons, and parents in poor condition are predicted to favor investment in daughters.

r/K selection theory, which, in ecology, relates to the selection of traits in organisms that allow success in particular environments. r-selected species, (in unstable or unpredictable environments), produce many offspring, each of which is unlikely to survive to adulthood, while K-selected species, (in stable or predictable environments), invest more heavily in fewer offspring, each of which has a better chance of surviving to adulthood.

Evolutionary psychology is based on the belief that, just like hearts, lungs, livers, kidneys, and immune systems, cognition has functional structure that has a genetic basis, and therefore has evolved by natural selection. Like other organs and tissues, this functional structure should be universally shared amongst a species, and should solve important problems of survival and reproduction. Evolutionary psychologists seek to understand psychological mechanisms by understanding the survival and reproductive functions they might have served over the course of evolutionary history.

While philosophers have generally considered human mind to include broad faculties, such as reason and lust, evolutionary psychologists describe EPMs as narrowly evolved to deal with specific issues, such as catching cheaters or choosing mates.

Some mechanisms, termed domain-specific, deal with recurrent adaptive problems over the course of human evolutionary history. Domain-general mechanisms, on the other hand, deal with evolutionary novelty.

EP argues that to properly understand the functions of the brain, one must understand the properties of the environment in which the brain evolved. That environment is often referred to as the environment of evolutionary adaptedness, or EEA for short.

The term environment of evolutionary adaptedness was coined by John Bowlby as part of attachment theory. It refers to the environment to which a particular evolved mechanism is adapted. More specifically, the EEA is defined as the set of historically recurring selection pressures that formed a given adaptation, as well as those aspects of the environment that were necessary for the proper development and functioning of the adaptation. In the environment in which ducks evolved, for example, attachment of ducklings to their mother had great survival value for the ducklings. Because the first moving being that a duckling was likely to see was its mother, a psychological mechanism that evolved to form an attachment to the first moving being would therefore properly function to form an attachment to the mother. In novel environments, however, the mechanism can malfunction by forming an attachment to a dog or human instead.

Humans, comprising the genus Homo, appeared between 1.5 and 2.5 million years ago, a time that roughly coincides with the start of the Pleistocene 1.8 million years ago. Because the Pleistocene ended a mere 12,000 years ago, most human adaptations either newly evolved during the Pleistocene, or were maintained by stabilizing selection during the Pleistocene. Evolutionary psychology therefore proposes that the majority of human psychological mechanisms are adapted to reproductive problems frequently encountered in Pleistocene environments.[11] In broad terms, these problems include those of growth, development, differentiation, maintenance, mating, parenting, and social relationships. To properly understand human mating psychology, for example, it is essential to recognize that in the EEA (as now) women got pregnant and men did not.

If humans are mostly adapted to Pleistocene environments, then some psychological mechanisms should occasionally exhibit “mismatches” to the modern environment, similar to the attachment patterns of ducks. One example is the fact that although about 10,000 people are killed with guns in the US annually,[12] whereas spiders and snakes kill only a handful, people nonetheless learn to fear spiders and snakes about as easily as they do a pointed gun, and more easily than an unpointed gun, rabbits or flowers.[13] A potential explanation is that spiders and snakes were a threat to human ancestors throughout the Pleistocene, whereas guns (and rabbits and flowers) were not. There is thus a mismatch between our evolved fear learning psychology and the modern environment.[14][15]

Evolutionary psychologists use several methods and data sources to test their hypotheses, as well as various comparative methods to test for similarities and differences between: humans and other species, males and females, individuals within a species, and between the same individuals in different contexts. They also use more traditional experimental methods involving, for example, dependent and independent variables.

Evolutionary psychologists also use various sources of data for testing, including archeological records, data from hunter-gather societies, observational studies, self-reports, public records, and human products.[16]

Applying evolutionary theory to animal behavior is uncontroversial. However, adaptationist approaches to human psychology are contentious, with critics questioning the scientific nature of evolutionary psychology, and with more minor debates within the field itself.[17][18] Criticisms of the field have also been addressed by scholars.[19]